Look for a sign-up Gmail. If you can login your email account, search for a welcome email from Google or Gmail. If you are not able to find the welcome message, you should check for the verification email on another Gmail account that you used then.

Ask your Gmail contacts. If you are using Gmail, ask friends and family when they first got an email messages from your Gmail account.

Think about related actions or intakes. Try to find out if you created your Gmail account when you:

Had an important life event, like graduating completing school education or something else.

Bought a new mobile phone computer or any other device. Check for any receipts or contracts that could be helpful to find out.

If you are not able to find out the date, you should try your best guess.